Radar sensor systems are frequently used in driver assistance systems for motor vehicles, for example, for an automatic distance regulation or for early detection of the risk of a collision.
Precipitation in the form of rain or spray splashed up from the road may reflect a portion of the emitted radar beam, thus causing the range of the radar beam and accordingly the locating depth of the radar sensor to be reduced. For purposes of traffic safety it is important that such a limitation of the function of the radar sensor system can be detected as immediately as possible.
German Patent Application No. DE 10 2006 054 320 A1 describes a method for a multi-beam radar, specifically for an FMCW radar sensor system. In this method, the locating signals of multiple radar beams are each integrated separately and the resulting integrals are compared with one another.
The functional principle of an FMCW radar sensor system (frequency modulated continuous wave) is that the radar signal is sent continuously; however, the frequency of this signal is modulated periodically using up and down ramps (the term “ramp” in this case is not intended to mean that the frequency modulation must necessarily be linear within the “ramp.”) A mixer mixes a portion of the transmission signal with the signal received from the antenna and thus produces a mixed product, the frequency of which corresponds with the difference between the frequency of the instantaneous transmission signal and the frequency of the received signal.
When a radar echo is received from a located object, the frequency of the mixed product is thus a function of the signal transit time and accordingly the distance of the object but also of the relative velocity of the reflecting object due to the Doppler Effect. In the spectrum formed from the mixed product, each located object is therefore distinguished on each modulation ramp as a peak in the frequency as a function of the distance and the relative velocity. By comparing the frequency positions of peaks—originating from the same object—on modulation ramps having a varying gradient, it is possible to determine the distance and the relative velocity of the object.
Raindrops or splashed up spray also represent “objects” in this sense, which leave behind a weak but nonetheless still detectable peak in the spectrum at distances from the radar sensor that are not too great, for example at distances up to about 10-50 m. In heavy precipitation, these peaks add up in the frequency range corresponding to the distance range referred to above to form a background signal, the so-called rain clutter. The output of this rain clutter is thus a measure of the presence and intensity of precipitation.
However, if “genuine” objects such as preceding vehicles are present in the locating range of the radar sensor and within the distance interval across which integration is performed, such vehicles producing a significantly more pronounced peak in the spectrum, the integral is falsified by these peaks so that it may no longer be used as a reliable indicator of precipitation. Therefore, it is only possible to use the conventional method in scenarios in which no genuine objects are present in the locating range in question.